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Fact Sheets

This fall, for the first time, NASA plans to test a laser-based sensor in space that will help scientists better understand global climate and how it might be changing.

The instrument, called LITE (Lidar In-Space Technology Experiment), will orbit the Earth while positioned inside the
payload bay of Space Shuttle Discovery. During this nine-day mission, LITE will measure the Earth's cloud cover and track
various kind of particles in the atmosphere. Designed and built at the NASA Langley Research Center, LITE is the first use of a lidar
(light detection and ranging) system for atmospheric studies from space.

Lidar is similar to the radar commonly used to track everything from airplanes in flight to thunder-storms. But instead of bouncing
radio waves off its target, lidar uses short pulses of laser light. Some of that light reflects off of tiny particles in the atmosphere
and back to a telescope aligned with the laser. By precisely timing the lidar "echo," and by measuring how much laser light is received by the telescope, scientists can accurately determine the location, distribution and nature of the particles. The result is a
revolutionary new tool for studying constituents in the atmosphere, from cloud droplets to industrial pollutants, that are difficult to
detect by other means.

Why lasers?

Laser light, directed toward Earth, will bounce off of thin clouds, dust particles and the surface of the Earth LITE's telescope receiver will "catch" that reflected light.)

Virtually all remote sensing satellites, including the ones that
produce our daily weather maps, rely on passive sensing. They
simply measure the amount of solar radiation -- visible light or
other wavelengths -- reflected, not emitted, back to the satellite
from clouds, ocean or solid land. Lidar is an active sensor. It
carries its own source of laser illumination, which means that it
can determine where and when it will make measurements, both in the
daytime and at night.

Lasers also produce a tight, coherent beam that does not
disperse as it travels away from its source, as ordinary light
does. From an orbital altitude of about 140 nautical miles (260
km), LITE's pencil-wide laser beam would spread to approximately
300 meters (984 ft or 328 yards) wide at the surface -- about the size of three football
fields. This allows the LITE instrument to measure a very small,
narrowly defined column of the atmosphere with each pulse. A
space-based lidar offers another great advantage in its ability to
penetrate thin or broken clouds to "see" through to the troposphere
(i.e., the lower part of the atmosphere where weather systems
develop). From its vantage point above the atmosphere, LITE's lidar
instrument -- the most powerful civilian laser ever flown in space
-- will flash extremely short pulses of laser light directly
downward, ten times every second. These pulses, lasting less than
30 billionths of a second each, will be in three different
wavelengths corresponding to ultraviolet, infrared and visible
green light. Because the wavelengths are very precisely known, and
because LITE's telescope is designed to filter out other types of
radiation, the signals returning to the Space Shuttle after
reflecting off of small airborne dust particles, water droplets and
other aerosols (suspended particles) are easy to identify. Timing
the returned signal pinpoints the particleͳ altitude to within
an accuracy of 15 meters (approx. 49 feet).

Lidar's ability to locate water droplets and ice particles in
the atmosphere gives scientists a powerful new tool for studying
clouds. Some high, thin cirrus clouds are invisible to conventional
remote sensing satellites. LITE will be able to determine their
heights with great precision. It also will reveal the vertical
structure of complex, multi-layered clouds that contain different
sizes and types of particles at different altitudes. The interiors
of these clouds would be hidden from ordinary, passive sensors.

Scientists also will be able to gain an estimate of density and
temperature variations within the stratosphere (the region where
most of our ozone resides, located about 6 to 30 miles (9.7 to 48 km) above the
Earth) by studying the returning lidar signals at LITE's
ultraviolet wavelength. In the stratosphere, LITE will be able to
map with unprecedented accuracy the particles produced by violent
volcanic eruptions. These particles help to explain global
circulation and are important to understanding climate. In
addition, LITE will return valuable data on the planetary boundary
layer close to the Earth's surface, where the atmosphere interacts
with the ocean and solid land, and where much of the dust and
pollutants in the atmosphere reside. Finally, LITE will determine
reflection characteristics of the Earth's surface. This data will
be used both to determine LITE's ability to measure vegetative
cover and to distinguish various types of surfaces.

Why space?

Ground-based lidar instruments can profile the atmosphere over a
single viewing site, while lidars onboard aircraft can gather
upward- or downward-looking data over a wider area. But each of
these methods is limited to sampling a comparatively small region.
A space-based lidar offers a truly global view. Space Shuttle
Discovery, orbiting at an inclination of 57 degrees to the equator,
will pass over about 10,000 miles (16,093 km) of the planet's surface every 90
minutes. The LITE instrument can therefore collect data for a wide
range of geographic and atmospheric settings, including remote
areas like the open ocean, in a very short period of time.

The LITE mission

A NASA Langley scientist prepares LITE for the September 1994 mission.

The LITE instrument will be mounted to a pallet inside the open payload bay of Discovery, which will orbit upside-down positioning LITE toward the Earth. Discovery will fly at a relatively low altitude (about 140 nautical miles or 260 km), so that each downward-pointing lidar pulse is dispersed as little as possible on its way down through the atmosphere. Over the course of
its nine-day mission, LITE will collect atmospheric data during ten separate periods lasting 4.5 hours each. During those periods, the
returning lidar signals collected by LITE's telescope will be converted to digital data, which will be stored on tape and
simultaneously transmitted to investigators on the ground.

In addition, the LITE instrument will take a number of 15-minute "snapshots" over target areas selected either for scientific
interest or to support validation observations. These validation observations will involve instruments at ground stations, on
balloons, and on aircraft, all of which will gather data to help calibrate the LITE results. A lidar at the Langley Research Center,
for example, will take upward-looking data at the exact time the Space Shuttle is passing overhead. Among the other "snapshot"
targets are sites in Europe, Australia, and the Sahara desert (to observe desert dust).

Another experiment requires that the Shuttle execute roll and
pitch maneuvers to change the angle at which the lidar reflects off
of its targets below. These tests will be useful to engineers
designing future lidar instruments that can scan from side to side
or front to back instead of holding to a fixed, downward-looking
point of view.

What will scientists learn from the LITE experiment?

Laser light from the shuttle contacts thin clouds, dust particles and the Earth's surface, and a reflection is bounced back to LITE's telescope.

Because this type of lidar has never flown in space, the LITE
mission is primarily a technology test. Scientists and engineers
want to verify that the entire system works as planned in orbit,
for example, that the laser and telescope remain aligned, that the
built-in cooling system can handle the heat produced by a powerful
lidar instrument and that the signals and noises are measured as
expected. The Space Shuttle is an ideal "platform" for conducting
this kind of technology test. It provides the opportunity to fly a
heavy, multi-purpose instrument at comparatively low cost without
building a dedicated satellite. Then, once the practical utility of
lidar in space is demonstrated, the lessons learned during the LITE
mission can be applied to designing future, operational systems
that are lighter in weight, use less spacecraft power and are more
capable.

Eventually, lidar instruments could be flown on permanently
orbiting satellites to provide continuous global data. While LITE
will collect data on a wide range of particles, from aerosols in
the stratosphere to cloud droplets and pollutants, future lidar
instruments could be tailored to specific purposes. While one
instrument studied clouds, another could track urban smog or desert
dust storms.

Perhaps the greatest value of early space-based lidar is the
unprecedented accuracy with which it can measure clouds on a global
scale. Information on clouds is critical to improving computer
models of global climate. Current remote sensing satellites leave
large gaps in our understanding of how clouds reflect and absorb
solar energy, and how heat and moisture are exchanged between the
air, ocean and land. Only by gathering more accurate information
can scientists improve their models to the point where they can
confidently predict the behavior of the real atmosphere, and tell
how the environment is being affected by human activity. LITE - and
its successors - will make a unique and valuable addition to that
store of information.

The laser NASA will use on this mission poses no hazard to the
general public. There is no hazard when viewing with the naked eye,
binoculars or small telescopes. In the interest of safety, the
International Science community has been asked not to attempt to
view the Shuttle directly through any telescope larger than 6
inches in diameter during the STS-64 mission.